1. Field of the Invention
The present invention is directed to an inverter AC power supply providing a high frequency AC voltage to a load, and more particularly to an inverter AC power supply composed of a chopper for providing a smoothed DC voltage from an AC voltage and an inverter for providing the high frequency AC voltage from the smoothed DC voltage.
2. Description of the Prior Art
An inverter AC power supply has been widely utilized in the art to drive such as discharge lamps and electric motors. Typical inverter power supplies are proposed in U.S. Pat. No. 4,933,831 and Japanese Patent Early Publication (KOKAI) No. 2-20366, each configured to includes a chopper of providing a smoothed DC voltage from an input AC voltage and an inverter providing a high frequency AC voltage from the smoothed DC voltage. In these prior power supplies a special design is made to share switching elements and diodes for the chopper and the inverter in order to simplify the circuit with an attendant reduction in the number of the components. However, U.S. Pat. No. 4,933,831 requires an external controller for alternately turning on and off the switching elements and therefore necessitates an additional circuitry which add complexity to the overall power supply circuit. Further, since the external controller is designed to turn on the switching elements for a fixed time period, there remains a problem that only a less input AC current is drawn to the chopper as an input pulsating AC voltage goes low, thereby failing to exactly conform an envelop of the input AC current to a sinusoidal waveform of the input AC voltage, and therefore lowering a power factor to that extent. On the other hand, the power supply of the Japanese Patent Publication No. 2-20366 is designed to eliminate the above problem by incorporating an external controller with an extra circuitry which monitors an input AC pulsating voltage and varies the turn-on period of the switching elements depending upon the monitored input AC voltage for improving the power factor. The power supply of this type therefore necessitates a complicated circuit arrangement due to not only the external controller but also the extra circuit with an attendant increase in the number of the components, which adds more cost and greater bulk to the assembled power supply and militates against the use in powering the load where the cost and bulk of primary concern.